How to Catch a Spiritomb in Pokemon Diamond

Spiritomb is a rare Ghost/Dark type Pokémon with no weaknesses. You encounter it by restoring the Hallowed Tower near Solaceon Town with the Odd Keystone, then talking to 32 people in the Underground. To go underground you need the Explorer Kit from the Underground Man in Eterna City. Because you may not catch Spiritomb on your first try, it's best to save right before the battle. If you fail, turn the game off without saving and try again. Try a Quick Ball first with Spiritomb to avoid a battle.

Instructions

Locate the "Hallowed Tower" on the right after the pit with the bicycle ramp in it. It looks like a pile of rocks with an opening in the middle.

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Place the "Odd Keystone" in that hole. If you don't have the Odd Keystone yet, go left (west) from the Hallowed Tower, across the piers, through Hearthome City and the passageway building to the west of Hearthome, to the grassy area (Rt. 208) where you'll encounter an Artist and a Karate Youth below him to the left in the trees. Battle the youth and he'll give you the keystone.

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Use the key item Explorer Kit to go to the Underground.

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Talk to 32 people in the Underground. The only way to do this is to have a friend with Pokemon Diamond, Pearl or Platinum in the same room with you and you go into the Underground together to talk to each other via DS wireless. You will need to go underground, greet your friend, then go back up 32 times, or go in and out of your underground Secret Base, then greet your friend, 32 times. To count as one of the 32, you must press the A button to greet your friend. He can't greet you.

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Go back up from the Underground and return to where you placed the Odd keystone in the Hallowed Tower.

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Press the A button, and a Spiritomb will battle you. He has no weaknesses, however, so you best be prepared with a Master Ball, a Quick ball or an Ultra Ball and healthy Pokémon to wear him down in battle in order to catch him.

Tips & Warnings

The Odd Keystone can also be obtained in Twinleaf Town. Surf on the water surface of the rectangular pond outside your house and use the Pokétch Item Finder to locate it as a hidden object near the middle tree on the left side.
Less commonly, digging Underground may turn up an Odd Keystone as well.
The Explorer Kit is obtained from the Underground Man in the building next to the Pokémon Center in Eterna City.